1. Field of the Invention
The present invention relates to a wireless sensor network. More particularly, the present invention relates to a network device for a wireless sensor network and data transmission method thereof that is capable of improving channel access efficiency and saves energy.
2. Description of the Related Art
Wireless networks built as Wireless Local Area Network (WLAN) and Wireless Personal Area Network (WPAN) are being widely deployed and the two types of networks coexisting with each other. Typically, a WLAN is implemented on the basis of the Institute of Electrical and Electronics Engineers (IEEE) 802.11x standards for supporting a relatively broad coverage area of 100 m, and a WPAN is implemented on the basis of the IEEE 802.15x standards. Several IEEE 802.15 standards, which includes Bluetooth, ZigBee, and Ultra Wideband (UWB), are currently either ratified or under development for use in wireless sensor networks. A wireless sensor network is composed of a plurality of spatially distributed sensor nodes. These sensor nodes share a single channel for transmitting data in active period. That is, the sensor nodes collect information in real time and transmit the information to a sink node in active periods.
In a case of IEEE 802.11 based WLAN, a network device performs carrier-sensing for checking whether or not the channel is idle. If the device determines that the channel is busy, the network device senses the channel until the channel is idle. The carrier-sensing is performed to detect the presence of ongoing transmissions by a Clear Channel Assessment (CCA) method. With the use of CCA method, the IEEE 802.11 WLAN is advantageous in channel access efficiency but not in energy utilization efficiency.
In an IEEE 802.15 based WPAN, however, a sensor node performs carrier-sensing once after the backoff period. If the sensor node detects that the channel is busy, the sensor node increases a contention window to twice the original size. The sensor node sets the contention window during the active period and then transmits the information during the contention period. That is, as a result of the carrier sensing, if the channel is idle, the sensor node transmits packets. On the other hand, if the channel is already occupied by another node or the previous transmission attempt fails, the sensor node retries the transmission with exponentially increased contention window size.
For this reason, the IEEE 802.15 WPAN is considered to be superior to the IEEE 802.11 WLAN in view of energy utilization efficiency. In the IEEE 802.15 WPAN, however, the sensor node recognizes the idle time of the channel and increases the contention window size exponentially in a conservative manner, resulting in low channel access efficiency.